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# pitch dia question

## pitch dia question

(OP)
So I have a set of gears

teeth:76
MOD: 3
Pressure Angle: 20 deg
pitch dia: 9.444
tooth angle: 18.1* (LH/RH)
gear lash: .003-.003
pin dia: .204
Over pins: 9.7269/9.7244

these are in a very old machine and the center distance is measuring: 9.460
we have a lot of other machines out there where the center distance is 9.444 so i cant change the angle to make the pitch dia work.

an engineer I work with wants to just change the pitch dia to 9.460" and the OD. to 9.696" and says we can leave the MOD and Angle the same. that you have to match the pitch dia to the center distance. I know ideally that is what you want to do. But from my understanding you can just arbitrarily change a pitch dia to match your center distance while leaving everything else the same.

I figured we should do a tooth correction. using a couple gear calculators this is the info I come up with.

teeth:76
MOD: 3
Pressure Angle: 20 deg
pitch dia: 9.444
tooth angle: 18.1* (LH/RH)
gear lash: .003-.003
pin dia: .204
correction factor: .0694
center distance: 9.460"

### RE: pitch dia question

Why don't you just change the helix angle to 18.40?

### RE: pitch dia question

(OP)
because we have a lot of machines out in the field with this gear set-up and there is actually (4) gears in the system so if we changed the angle if any machines went down we would have to replace all the gears.

if we end up making a specific set of gears for this machine I will just change the angle.

### RE: pitch dia question

But if you, as you said, change the correction factor to get to the required CD, you'll change the gears as well - so what's the difference?

### RE: pitch dia question

(OP)
you are right there is no difference, part of my question was asking if I do a tooth correction will it still mesh with the old gears. since this is the case I will more than likely have to create a new set of gears specifically for this machine in which case I will just adjust the tooth angle.

### RE: pitch dia question

tterb83,

Are these going to be custom gears?

Talk to your gear cutter. I was preloading gears to eliminate backlash. Our gear cutter told us he could cut the gears slightly oversize so that the action would be optimized for zero clearance. This may be a solution to your problem too.

--
JHG

### RE: pitch dia question

Can you live with more lash? You can have the 9.444 gears operating at a centre distance of 9.460 with more lash. They will mesh correctly, but would be a bit weaker. If you can't than you need special 9.460 gears for this one machine.
BTW, the calcs look to be correct.

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